The present invention is applicable to adjusting the particle size distribution of all kinds of particulates, including sands, ores, minerals, powdered metals, seeds and grains. The invention is especially useful in obtaining a controlled gradation of crushed fine aggregate produced from quarried stone by crushing or grinding. Crushed fine aggregate is referred to in the art by various terms such as stone sand, crusher sand, crushed fine aggregate, specification sand or manufactured sand. In this specification, such crushed fine aggregate is referred to as "stonesand". An accepted standard for stonesand used in concrete is set forth in Standard Specification C-33 for Concrete Aggregates as published by the American Society for Testing and Materials (ASTM). Stonesand may be produced from almost all rock types which are commonly quarried to make coarse aggregate for roadbeds and the like. As natural sand deposits become depleted or unavailable through land development, the demand for stonesand has increased in recent years.
There are basically two different types of crushers for the rock types yielding stonesand. Jaw, gyratory and cone crushing are compression types depending upon compression (squeezing), friction and/or attrition between particles to break down the larger rock particles. Roll, rod mill, hammer mill and centrifugal are impact types which rely largely upon impact (hitting) for breakage. Depending on the rock type, the impact crushers generally produce a more cubical shaped particle than the compression crushers. Only limited control of particle shape or size can be realized in a communition process, especially in the smallest sizes produced, because of the tendency of breakage to occur along the surfaces of weakness dictated by the mineralogy of the material being crushed. Regardless of the type of crusher used, stonesand tends to be somewhat deficient in the intermediate particle size classes (No. 30 to No. 100 mesh), relative to sands which will satisfy the ASTM C-33 specification and to contain more fracture dust or fines (minus 100 mesh) than natural sands. On the other hand, the fractured cubical shape of some stonesand is capable of providing a concrete of higher strength and greater durability (more resistant to freezing and thawing deterioration) than some natural sands which are more rounded in shape.
In order to obtain good quality stonesands, it is therefore often necessary to remove at least a portion of the minus 100 and minus 200 mesh material, as well as some of the larger sizes near 3/8 inch mesh. To accomplish this and improve the overall gradation of stonesand, some type of classifer is usually employed. Classifiers are also generally of two types, namely wet classifiers and dry classifiers. Classification, whether by wet or dry processes, is possibly the single most important step in the production of a stonesand product of acceptable quality. Although wet classification systems generally produce more reproducible particle size distributions, such systems are of relatively low capacity per unit of capital cost and are relatively expensive to operate. On the other hand, dry classification systems of the prior art require that the aggregate feed be adequately populated in the particle sizes of interest and be uniform in moisture content because any significant variations, particularly in moisture content, will result in an output that does not meet the needed criteria. Excessive moisture content may also cause blinding of screen classifiers such that the required degree of passage of undersize particles through the screen is prevented by partial or complete blockage of the screen apertures.
Conventional approaches to producing a graded stonesand product often involve separating the crushed feed material into individual size fractions and then recombining two or more of those fractions in the proportions necessary to obtain the relative quantities of each fraction desired in a final product. The multiple processing stages required by these prior art approaches are time consuming and are not energy efficient. The necessity for blending two or more fractions often causes problems in handling the particulates and in adequately mixing the different size fractions to achieve the required uniformity in the final product.
Conventional classification of particulates with multiple screens may be in the form of batch sieving or continuous screening. In batch sieving, a stacked set of sieves are operated so as to provide particle exposure to the screen for a relatively long period of time that permits passage of nearly all (typically greater than 99 mass percent) of the undersize particles, i.e., those of a size capable of passing through a given screen. This is referred to in this patent specification as operating under complete separation conditions. A set of sieves operated in this manner will separate the batch feed into mass fractions corresponding to different size classes, where each size class consists of all particle sizes between the mesh sizes of two successive sieves (or screens). Each such mass fraction represents the ratio of mass of particles in the given size class to the total mass of all particles in the sample of the parent size distribution. The sieving is carried out for the period of time required to achieve substantially complete separation of the feed material into preselected size classes. The mass fractions so separated will not be substantially changed by sieving for longer periods of time. The mass fractions provided by classifiers employing batch sieving may then be reblended in the desired proportions to provide a finished product having the size distribution desired for a given application. In continuous screening, the screen sizes and lengths are selected as if each screening stage were to be carried out in a fashion analogous to batch sieving but assuming a somewhat lesser degree of complete screening (typically 85 to 95 mass percent). The mesh size of the screen, the screen length, the screen vibratory rate and values of other screening parameters are therefore selected to provide the desired product by assuming a predetermined level of essentially complete screening chosen on the basis of the estimated characteristics of a constant particle size distribution of feed material under fixed conditions of screening. The 85 to 95% completion values for continuous screening typically arise because of the finite length of practical screens. Very long screens of impractical lengths would usually be required to achieve operation close to complete screening conditions (greater than 95 mass percent passage of those particles capable of passing through the screen).
In conventional continuous screening systems, which often operate relatively near complete screening conditions, it is desirable to control closely the screening conditions and the moisture content, size distribution and other characteristics of the feed because significant variations in feed and/or screening conditions can cause corresponding variations in the rate of passage of undersize particles through the screen apertures and result in a product outside the limits of the applicable size distribution specification. Typically these controls are not used and sometimes it is not even recognized that they should be used. In addition, conventional screening systems are often tailor-made for a given feed and set of screening conditions such that product specifications cannot be maintained with a significantly different feed or under significantly different screening conditions.
Prior art classifiers employing continuous screening processes depend upon essentially complete screening to provide the desired size distribution in the finished product. An example of one such prior art process is illustrated by U.S. Pat. No. 4,032,436 to Johnson, the entire contents of which are incorporated herein by reference. Such classifiers may be sensitive to screen blinding where a portion of the open screen area is blocked by near size particles. Variations in the rate of passage of undersize particles through the screen because of blinding may cause excessive waste and/or the finished product to be out of specification.
A specific application of stonesand, such as in making concrete or asphalt, may require a closely defined sieve analysis and fineness modulus (F.M.). In other words, the stonesand must be carefully processed so as to have a consistent gradation and a consistent F.M. as necessary to meet applicable specifications and achieve a high quality concrete or asphalt composition with good workability, flowability and finishability.
ASTM Standard Specification C-33 (ASTM C-33) as applied to stonesand has the following sieve analysis limits based on the cumulative percentages passing through each sieve size indicated upon screening substantially to completion: 100% passing 3/8 inch, 95 to 100% passing No. 4, 80 to 100% passing No. 8, 50 to 85% passing No. 16, 25 to 60% passing No. 30, 10 to 30% passing No. 50, 2 to 15% passing No. 100 and 0 to 7% passing No. 200. ASTM C-33 further requires that not more than 45% of the sample be retained between any two consecutive sieves, that the F.M. not be less than 2.50 nor more than 3.10 and that the F.M. not vary by more than 0.20 unless suitable adjustments are made in proportioning the concrete to compensate for the difference in grading. Thus, once the proportion of stonesand is selected for concrete, it is preferable that such fluctuations in the stonesand grading be prevented to avoid having to change this proportion.
To determine whether a stonesand product meets ASTM C-33, a sample of the product is subjected to a sieve analysis using batch sieving through a set of test sieves having the sizes specified above to measure the percent retained on each of the sieves. The F.M. value is then determined by summing the accumulated weight percentages retained on the successive sieves and the resulting number which is in excess of 100% is divided by 100 to produce a number which is the fineness modulus. A more detailed explanation of the F.M. indicator is set forth in the Johnson patent referenced above.